Catadioptric system for photolithography

ABSTRACT

Catadioptric systems are provided, comprising a first imaging system, a plane mirror, and a second imaging system. The first imaging system comprises a single-pass optical system and a double-pass optical system that further comprises a concave mirror and a double-pass lens group. The second imaging system comprises a plane mirror and an aperture. The first imaging system forms an intermediate image of an object; the second imaging system re-images the intermediate image on a substrate. The catadioptric systems satisfy various conditions.

FIELD OF THE INVENTION

The present invention pertains to catadioptric systems for photolithography and in particular, to high-resolution catadioptric systems for use with short-wavelength illumination.

BACKGROUND OF THE INVENTION

Catadioptric optical systems such as those disclosed in Japanese patent document 234722 (1992) and U.S. Pat. No. 4,779,966 have been developed to correct field curvature in photolithography systems. In these catadioptric systems, a first imaging system containing a concave mirror forms an intermediate image of a reticle. A plane mirror directs a light flux from the intermediate image to a second imaging system that re-images the intermediate image onto a wafer. These catadioptric systems have only one plane mirror and the reticle and the wafer are not parallel, thereby complicating synchronous scan of the reticle and the wafer. Other catadioptric systems have been developed wherein the second imaging system comprises a second plane mirror, permitting the reticle and the wafer to be parallel.

In general, catadioptric optical systems with tilted plane mirrors exhibit large aberrations unless the tilts of the plane mirrors are fixed precisely with respect to other portions of the optical system. In catadioptric systems that produce high-resolution images, the angular alignment tolerances of the plane mirrors must be very small and catadioptric system assembly is difficult.

SUMMARY OF THE INVENTION

Catadioptric systems according to the present invention have relaxed angular alignment tolerances for the plane mirrors or other reflecting surfaces that permit the reticle and wafer to be in parallel planes. Catadioptric systems according to the present invention preferably comprise, from objectwise to imagewise, a first imaging system comprising a single-pass optical system and a double-pass optical system, and a second imaging system. The double-pass optical system comprises, from objectwise to imagewise, a double-pass lens group and a concave mirror.

The single-pass optical system receives a light flux from an object, e.g., a reticle, and transmits the light flux to the double-pass optical system. The double-pass lens group receives the light flux from the single-pass optical system and transmits the light flux to the concave mirror. The concave mirror reflects the light flux back through the double-pass lens group and an intermediate image is formed. A first plane mirror or other reflector placed near the intermediate image reflects the light flux to the second imaging system and a image of the reticle in formed, generally on the surface of a wafer or other substrate.

The second imaging system preferably comprises a first lens group and a second lens group. In addition, the catadioptric systems preferably comprise a second plane mirror or other reflector and an aperture. The second plane mirror and the aperture preferably satisfy at least one of the conditions:

    L.sub.1 /L<0.1

    L.sub.1 /L.sub.2 <0.2

wherein L₁ is the axial distance from the second plane mirror to the aperture, L is the axial distance from the reticle, or other object, to the wafer, or other image location, and L₂ is the axial distance from the first plane mirror to the wafer W, or other image location.

The catadioptric systems preferably satisfy the condition:

    |L.sub.1 /f.sub.1 |<1.5

wherein f₁ is the focal length of the second lens group of the second imaging system.

It is further preferable that the catadioptric systems satisfy the condition:

    |β.sub.1 |<0.2

wherein β₁ is the magnification of the second lens group of the second imaging system.

The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of a preferred embodiment which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic optical diagram of a catadioptric system according to Example Embodiment 1.

FIGS. 2(a)-2(b) are plots of transverse aberrations exhibited by Example Embodiment 1. FIG. 2(a) depicts transverse aberrations at an image height of Y=18.6 mm. FIG. 2(b) depicts transverse aberrations at an image height of Y=5 mm.

FIG. 3 is a schematic optical diagram of a catadioptric system according to Example Embodiment 2.

FIGS. 4(a)-4(b) are plots of transverse aberrations exhibited by Example Embodiment 2. FIG. 4(a) depicts transverse aberrations at an image height of Y=18.6 mm. FIG. 4(b) depicts transverse aberrations at an image height of Y=5 mm.

FIG. 5 is a schematic elevational view of a projection-exposure apparatus comprising a catadioptric system according to the invention.

DETAILED DESCRIPTION

For convenience in describing the Example Embodiments, an axis is a line or connected series of line segments through a center of curvature of a reflecting or refracting surface. As is well-known, optical systems with tilted reflecting surfaces have optical axes consisting of connected line segments. Directions along an axis toward an object or an image are called "objectwise" and "imagewise," respectively.

With reference to FIG. 1 and FIG. 3, for example, catadioptric systems according to Example Embodiments 1-2, respectively, project a demagnified image of a reticle R, or other object, onto a wafer W, or other substrate. The catadioptric systems preferably comprise, from objectwise to imagewise, a first imaging system A that forms an intermediate image of the reticle R, a first plane mirror M₁ placed near the intermediate image, and a second imaging system B that re-images the intermediate image onto the wafer W.

The first imaging system A preferably comprises, from objectwise to imagewise, a single-pass optical system Al and a double-pass optical system A₂. The double-pass optical system A₂ preferably comprises, from the single-pass optical system A₁ and along an axis, a double-pass lens group A_(2L) and a concave mirror M_(c). The double-pass lens group A_(2L) preferably comprises a concave lens L_(c) placed axially adjacent the concave mirror M_(c).

The second imaging system B preferably comprises, from objectwise to imagewise, a first lens group B₁ and a second lens group B₂. The second imaging system further comprises an aperture S. In addition, the second imaging system comprises a second plane mirror M₂ between the first lens group B₁ and the second lens group B₂. In both Example Embodiments 1-2, the second plane mirror M₂ is objectwise of the aperture S, but the second plane mirror M₂ can be imagewise of the aperture S.

The single-pass optical system A₁ receives a light flux from the reticle R and directs the light flux to the double-pass optical system A₂. The double-pass lens group A_(2L) transmits the light flux to the concave mirror M_(c). The concave mirror M_(c) receives the light flux and reflects the light flux back through the double-pass lens group A_(2L). The light flux then exits the double-pass optical system A₂ and is directed by the first plane mirror M₁ to the second imaging system B.

The first lens group B₁ of the second imaging system B receives the light flux reflected by the first plane mirror M₁ and directs the light flux to the second plane mirror M₂. The second plane mirror M₂ then reflects the light flux to the second lens group B₂ and an image is formed on the wafer W.

The catadioptric systems of the preferred embodiments are suitable for photolithography systems in which the reticle R is scanned through an off-axis, slit or arc-shaped illumination region. Patterns from the entire reticle R are projected onto the wafer W by synchronously scanning the wafer W while projecting patterns from the reticle R as the reticle R scans through the illumination region.

The second plane mirror M₂ and the aperture S are preferably arranged to satisfy at least one of the following Conditional Expressions 1-2:

    L.sub.1 /L<0.1                                             (1)

    L.sub.1 /L.sub.2 <0.2                                      (2)

wherein L₁ is the axial distance from the second plane mirror M₂ to the aperture S, L is the axial distance from the reticle R, or other object, to the wafer W, or other image location, and L₂ is the axial distance from the first plane mirror M₁ to the wafer W, or other image location. Because the second plane mirror M₂ is near the aperture S, the light flux reflected by the second plane mirror M₂ is substantially collimated. Off-axis aberrations such as coma, astigmatism, and distortion due to tilt of the second plane mirror M₂ are therefore reduced and tilt tolerances for the second plane mirror M₂ are relaxed. If Conditional Expressions 1-2 are not satisfied, then off-axis aberrations are unacceptably large.

Also, the catadioptric systems preferably satisfy Conditional Expression 3:

    |L.sub.1 /f.sub.1 |<1.5                  (3)

wherein f₁ is the focal length of the second lens group B₂ of the second imaging system B. Satisfying Conditional Expression 3 reduces aberrations in the second imaging system B. If Conditional Expression 3 is not satisfied, then f₁ is too small and correction of aberrations in the second imaging system B is difficult.

It is further preferable that Conditional Expression 4 be satisfied:

    |L.sub.1 /f.sub.1 |<1.5                  (4)

wherein β₁ is the magnification of the second lens group B₂ of the second imaging system B. Satisfying Conditional Expression 4 provides that the light flux at the second plane mirror M₂ is substantially collimated, i.e. an angular diameter of the light flux is less than the object-side numerical aperture. If Conditional Expression 4 is not satisfied, then the light flux has a large angular diameter and the second plane mirror M₂ tends to reflect the light flux non-uniformly.

Tables 1-2 contain specifications for the catadioptric systems of Example Embodiments 1-2, respectively. In Tables 1-2, the first column lists surfaces numbered in order from objectwise to imagewise along an axis along which the light flux from the reticle R propagates; the second column, labeled "r, " lists corresponding curvature radii; the third column, labeled "d, " lists axial separations between adjacent, imagewise surfaces; the fourth column, labeled "Material, " lists lens materials; and the fifth column, labeled "Group, " lists the group number of the surfaces. In the fifth column, a "*" indicates a surface of the double-pass optical system A2 encountered by a light flux after reflection from the concave mirror C_(m). Refractive indices of fused silica (SiO₂) and fluorite (CaF₂) at a wavelength of 193 nm are n=1.56019 and n=1.50138, respectively.

Table 3 contains values of the Conditional Expressions 1-4 and values of L₁, L, L₂, and f1 for Example Embodiments 1-2.

With reference to FIG. 2 and FIG. 4, it is apparent that the catadioptric systems are well-corrected for aberrations over a wavelength range of 193.4 nm±0.1 nm at image heights of Y=18.6 mm and Y=5 mm.

                  TABLE 1                                                          ______________________________________                                         (Example Embodiment 1)                                                         Surf.  r            d                                                          No.    (mm)         (mm)    Material  Group                                    ______________________________________                                         0      --           49.998            R                                        1      369.115      18.000  SiO.sub.2 A.sub.1                                  2      245.893      0.500                                                      3      227.674      33.705  CaF.sub.2 A.sub.1                                  4      -373.082     18.803                                                     5      -324.258     20.532  SiO.sub.2 A.sub.1                                  6      332.817      1.674                                                      7      340.581      20.389  SiO.sub.2 A.sub.1                                  8      604.750      27.395                                                     9      ∞      35.900  SiO.sub.2 A.sub.1                                  10     ∞      16.943                                                     11     391.176      30.000  CaF.sub.2 A.sub.2                                  12     -982.727     6.592                                                      13     -417.793     20.000  SiO.sub.2 A.sub.2                                  14     -1216.731    261.353                                                    15     478.547      40.000  CaF.sub.2 A.sub.2                                  16     -908.632     11.323                                                     17     325.213      20.000  SiO.sub.2 A.sub.2                                  18     208.331      48.917                                                     19     -196.257     20.000  SiO.sub.2 A.sub.2                                  20     1370.871     0.500                                                      21     430.209      42.793  CaF.sub.2 A.sub.2                                  22     -366.694     61.625                                                     23     247.465      25.000  SiO.sub.2 A.sub.2                                  24     286.274      68.753                                                     25     508.228      40.000  SiO.sub.2 A.sub.2                                  26     -930.828     27.931                                                     27     -313.824     25.000  SiO.sub.2 A.sub.2                                  28     -1017.267    19.454                                                     29     -276.064     25.000  SiO.sub.2 A.sub.2                                  30     1335.454     32.821                                                     31     -360.416     32.821            A.sub.2 (M.sub.c)                        32     1335.454     25.000  SiO.sub.2 A.sub.2*                                 33     -276.064     19.454                                                     34     -1017.267    25.000  SiO.sub.2 A.sub.2*                                 35     -313.824     27.931                                                     36     -930.828     40.000  SiO.sub.2 A.sub.2*                                 37     508.228      68.753                                                     38     286.274      25.000  SiO.sub.2 A.sub.2*                                 39     247.465      61.625                                                     40     -366.694     42.793  CaF.sub.2 A.sub.2*                                 41     430.209      0.500                                                      42     1370.871     20.000  SiO.sub.2 A.sub.2*                                 43     -196.257     48.917                                                     44     208.331      20.000  SiO.sub.2 A.sub.2*                                 45     325.213      11.323                                                     46     -908.632     40.000  CaF.sub.2 A.sub.2*                                 47     478.547      261.353                                                    48     -1216.731    20.000  SiO.sub.2 A.sub.2*                                 49     -417.793     6.592                                                      50     -982.727     30.000  CaF.sub.2 A.sub.2*                                 51     391.176      1.943                                                      52     ∞      236.637           M.sub.1                                  53     471.443      36.090  CaF.sub.2 B.sub.1                                  54     -1089.261    3.979                                                      55     306.858      20.000  SiO.sub.2 B.sub.1                                  56     247.195      312.806                                                    57     812.165      25.000  SiO.sub.2 B.sub.1                                  58     2628.418     145.000                                                    59     ∞      145.508           M.sub.2                                  60     -1094.809    30.000  SiO.sub.2 B.sub.2                                  61     1598.936     30.114                                                     62     --           81.437            S                                        63     -266.544     45.218  CaF.sub.2 B.sub.2                                  64     2115.935     0.550                                                      65     -213.134     30.096  SiO.sub.2 B.sub.2                                  66     -642.205     15.142                                                     67     1328.716     30.000  SiO.sub.2 B.sub.2                                  68     -654.044     1.236                                                      69     -210.004     45.167  SiO.sub.2 B.sub.2                                  70     -304.557     19.703                                                     71     -166.497     45.000  SiO.sub.2 B.sub.2                                  72     -72.336      6.218                                                      73     -71.786      66.262  SiO.sub.2 B.sub.2                                  74     2042.086     17.000                                                     75     --                             W                                        ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                         (Example Embodiment 2)                                                         Surf.  r            d                                                          No.    (mm)         (mm)    Material Group                                     ______________________________________                                         0      --           60.000           R                                         1      -210.000     18.000  SiO.sub.2                                                                               A.sub.1                                   2      -233.058     1.734                                                      3      301.818      32.109  CaF.sub.2                                                                               A.sub.1                                   4      -415.393     19.449                                                     5      154862.242   15.248  SiO.sub.2                                                                               A.sub.1                                   6      -528.109     5.460                                                      7      -316.309     18.000  SiO.sub.2                                                                               A.sub.1                                   8      275.570      74.064                                                     9      342.313      48.000  CaF.sub.2                                                                               A.sub.2                                   10     -248.024     1.806                                                      11     -250.000     20.000  SiO.sub.2                                                                               A.sub.2                                   12     3438.110     286.849                                                    13     390.013      40.000  CaF.sub.2                                                                               A.sub.2                                   14     -2017.162    22.849                                                     15     421.041      20.000  SiO.sub.2                                                                               A.sub.2                                   16     230.317      47.916                                                     17     -222.542     20.000  SiO.sub.2                                                                               A.sub.2                                   18     988.626      7.270                                                      19     11949.023    27.617  CaF.sub.2                                                                               A.sub.2                                   20     -328.913     0.500                                                      21     365.306      42.285  SiO.sub.2                                                                               A.sub.2                                   22     -1713.365    160.144                                                    23     -283.704     30.000  SiO.sub.2                                                                               A.sub.2                                   24     1076.349     30.701                                                     25     -353.136     30.701           A.sub.2 (M.sub.c)                         26     1076.349     30.000  SiO.sub.2                                                                               A.sub.2*                                  27     -283.704     160.144                                                    28     -1713.365    42.285  SiO.sub.2                                                                               A.sub.2*                                  29     365.306      0.500                                                      30     -328.913     27.617  CaF.sub.2                                                                               A.sub.2*                                  31     -11949.023   7.270                                                      32     988.626      20.000  SiO.sub.2                                                                               A.sub.2*                                  33     -222.542     47.916                                                     34     230.317      20.000  SiO.sub.2                                                                               A.sub.2*                                  35     421.041      22.849                                                     36     -2017.162    40.000  CaF.sub.2                                                                               A.sub.2*                                  37     390.013      286.849                                                    38     3438.110     20.000  SiO.sub.2                                                                               A.sub.2*                                  39     -250.000     1.806                                                      40     -248.024     48.000  CaF.sub.2                                                                               A.sub.2*                                  41     342.313      4.064                                                      42     ∞      180.000          M.sub.1                                   43     506.214      34.041  CaF.sub.2                                                                               B.sub.1                                   44     -256.332     3.017                                                      45     -450.000     20.000  SiO.sub.2                                                                               B.sub.1                                   46     -1453.242    422.966                                                    47     ∞      150.000          M.sub.2                                   48     -285.380     30.000  SiO.sub.2                                                                               B.sub.2                                   49     -954.824     50.000                                                     50     --           78.332           S                                         51     -220.000     45.000  CaF.sub.2                                                                               B.sub.2                                   52     -2665.536    6.535                                                      53     -200.000     27.411  SiO.sub.2                                                                               B.sub.2                                   54     -516.467     18.844                                                     55     632.373      30.000  SiO.sub.2                                                                               B.sub.2                                   56     -1060.585    19.112                                                     57     -553.788     45.000  SiO.sub.2                                                                               B.sub.2                                   58     5823.302     0.500                                                      59     -153.299     45.000  SiO.sub.2                                                                               B.sub.2                                   60     -120.000     1.243                                                      61     -125.615     66.000  SiO.sub.2                                                                               B.sub.2                                   62     3036.218     17.000                                                     63     --                            W                                         ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                         Values of Conditional Expressions                                              Variables and                                                                               Example    Example                                                Conditions   Embodiment 1                                                                              Embodiment 2                                           ______________________________________                                                L.sub.1 (mm)                                                                             205.6      230.0                                                     L (mm)    3287.2     3150.0                                                    L.sub.2 (mm)                                                                             1388.2     1290.0                                                    f.sub.1 (mm)                                                                             192.0      207.5                                              (1)    L.sub.1 /L                                                                               0.06       0.07                                               (2)    L.sub.1 /L.sub.2                                                                         0.15       0.18                                               (3)    |L.sub.1 /f.sub.1 |                                                    1.07       1.11                                               (4)    |β.sub.1 |                                                        0.1409     0.1932                                             ______________________________________                                    

In Example Embodiment 1, the second mirror M₂ is placed 205.6 mm objectwise of the aperture S; in Example Embodiment 2, the second mirror M₂ is 230 mm objectwise of the aperture S. As a result, coma, astigmatism, and distortion due to tilt of the plane mirror M₂ are small, thereby simplifying the fabrication of a high-resolution catadioptric system. In addition, irregular reflectance of the second mirror M₂ is reduced.

In both Example Embodiments 1-2, the second plane mirror M₂ is objectwise of the aperture S. However, the second plane mirror M₂ can be imagewise of the aperture S so long as Conditional Expressions 1-4 are satisfied.

A representative projection-exposure (microlithography) apparatus 10 comprising a catadioptric system as described above is schematically illustrated in FIG. 5. The apparatus 10 comprises an illumination optical system 12, a reticle or "mask" 14 defining a pattern for projection, a reticle stage 16 for holding the reticle, a catadioptric system 18 as described above, a photosensitive substrate 20 onto which the catadioptric system projects an image of the pattern defined by the mask 14, a substrate stage 22 for holding the substrate 20, a driver 24 for moving the reticle stage 16 as required during or between exposures, a driver 26 for moving the substrate stage 22 as required during or between exposures, and a controller 28 for controlling operation of the entire apparatus 10.

Having illustrated and demonstrated the principles of the invention in example embodiments, it should be apparent to those skilled in the art that the example embodiments can be modified in arrangement and detail without departing from such principles. For example, other reflecting elements such as prisms can be substituted for the first plane mirror M₁ or the second plane mirror M₂ of Example Embodiments 1-2. I claim as the invention all that comes within the scope of these claims. 

What is claimed is:
 1. A catadioptric system for projecting an image of an object onto a substrate, the catadioptric system comprising, from objectwise to imagewise:(a) a first imaging system having an optical axis and comprising a single-pass optical system and a double-pass optical system, the double-pass optical system comprising a concave mirror and a double-pass lens group, a light flux from the object being received by the single-pass optical system and directed to the double-pass lens group, the double-pass lens group directing the light flux to the concave mirror that reflects the light flux back through the double-pass lens group, thereby forming an intermediate image of the object; (b) a first plane reflector receiving the light flux from the double-pass lens group; (c) a second imaging system receiving the light flux from the first plane reflector and forming an image of the object on the substrate, the second imaging system comprising, from objectwise to imagewise, a first lens group, a second plane reflector, and a second lens group, wherein an aperture is disposed in the second lens group and the second plane reflector is disposed between the first lens group and the second lens group; (d) the catadioptric system satisfying at least one of the following conditions:

    L.sub.1 /L<0.1

    L.sub.1 /L.sub.2 <0.2

wherein L₁ is an axial distance from the second plane reflector to the aperture, L is an axial distance from the object to the substrate, and L₂ is an axial distance from the first plane reflector to the substrate.
 2. The catadioptric system of claim 1, further satisfying the condition:

    |β.sub.1 |<0.2

wherein β₁ is a magnification of the second lens group of the second imaging system.
 3. The catadioptric system of claim 1, further satisfying the condition:

    |L.sub.1 /f.sub.1 |<1.5

wherein f₁ is a focal length of the second lens group of the second imaging system.
 4. The catadioptric system of claim 3, further satisfying the condition:

    |β.sub.1 |<0.2

wherein β₁ is a magnification of the second lens group of the second imaging system.
 5. A catadioptric system for projecting an image of an object onto a substrate, the catadioptric system comprising, from objectwise to imagewise:(a) a first imaging system comprising a single-pass optical system and a double-pass optical system, the double-pass optical system comprising a concave mirror and a double-pass lens group, a light flux from the object being received by the single-pass optical system and directed to the double-pass lens group, the double-pass lens group directing the light flux to the concave mirror that reflects the light flux back through the double-pass lens group, thereby forming an intermediate image of the object; (b) a first plane reflector receiving the light flux from the double-pass lens group; (c) a second imaging system receiving the light flux from the first plane reflector and forming an image of the object on the substrate, the second imaging system comprising a second plane reflector and an aperture; (d) the catadioptric system satisfying at least one of the following conditions:

    L.sub.1 /L<0.1

    L.sub.l /L.sub.2 <0.2

wherein L₁ is an axial distance from the second plane reflector to the aperture, L is an axial distance from the object to the substrate, and L₂ is an axial distance from the first plane reflector to the substrate.
 6. A projection-exposure apparatus, comprising a catadioptric system as recited in claim
 1. 7. A projection-exposure apparatus, comprising a catadioptric system as recited in claim
 5. 